1. Field of the Invention
The present invention relates to a sheet feeder having a sheet cassette and an image forming apparatus such as a copying machine and a printer having the sheet feeder.
2. Description of the Related Art
In the related art, a sheet feeder used in an image forming apparatus such as a copying machine and a printer has a device capable of separately feeding every single sheet using a retard roller and a feed roller facing each other.
Such a retard roller separation type sheet feeder includes a pickup roller that picks up the sheet, a feed roller rotated in the same direction as that of the pickup roller and provided in a downstream side of a sheet feeding direction, and a retard roller that presses the feed roller with a predetermined pressing force (hereinafter, referred to as a retard pressure). In addition, a sheet is separated in a nip portion (hereinafter, referred to as a roller nip portion) between the feed roller and the retard roller.
The retard roller receives a certain torque in an anti-sheet feeding direction opposite to the sheet feeding direction through a torque limiter so as to be rotated in either the sheet feeding direction or the anti-sheet feeding direction. In addition, in a case where two or more sheets enter the roller nip portion, the retard roller is rotated in the anti-sheet feeding direction, so that it is possible to prevent overlapping feeding of the sheets, in which two or more sheets are overlappingly fed (refer to Japanese Patent Laid-Open No. 04-286558).
However, in the retard roller separation technique including the feeder disclosed in Japanese Patent Laid-Open No. 04-286558, it is difficult to separately feed every single sheet having a cutting failure portion. Here, the cutting failure portion refers to a portion where folding, bending, bulging, or the like is generated in an edge of the cut sheet when a sheet cut transverse section is stretched by a blade used to cut the sheet along a cut direction in the sheet cutting. Since the sheet cutting is performed for a plurality of overlapping sheets, there is a high possibility that a cutting failure portion is generated in the same position of the sheets.
Specifically, the cutting failure portion in front and rear edges of the sheet in the sheet feeding direction can be classified into four types: (1) a state that an upward cutting failure portion exists in a rear end side of the sheet conveying direction; (2) a state that a downward cutting failure portion exists in a leading end side of the sheet conveying direction; (3) a state that a downward cutting failure portion exists in a rear end side of the sheet conveying direction; and (4) a state that an upward cutting failure portion exists in a leading end side of the sheet conveying direction.
Subsequently, a sheet separation state in the states (1) to (4) will be described. Here, an upper sheet refers to a sheet that makes contact with a pickup roller, and a lower sheet refers to a sheet fed to the roller nip portion by virtue of a frictional force from the upper sheet, a resistant force of the cutting failure portion, and the like when the upper sheet is fed by the pickup roller.
In the sheet having two types of cutting failure portions described in the states (1) and (2), the cutting failure portions of the upper and lower sheets are not engaged with each other when the retard roller is rotated in the anti-sheet feeding direction, and the lower sheet moves in the anti-sheet feeding direction. For this reason, the upper and lower sheets are separated in the roller nip portion and are fed one by one.
Meanwhile, in the sheet having two types of cutting failure portions described in the states (3) and (4), the cutting failure portions of the upper and lower sheets are engaged with each other when the retard roller is rotated in the anti-sheet feeding direction, and the lower sheet moves in the anti-sheet feeding direction. For this reason, the upper and lower sheets are resistant to each other when they are separated in the roller nip portion, so that frequency of overlapping feeding increases, in which the sheets are overlappingly fed without separation.